Field of the Invention
Embodiments of the present invention generally relate to power conversion and, more particularly, to impedance matching in virtual impedance droop controlled power conditioning units (PCUs).
Description of the Related Art
Droop control is an industry standard technique for autonomously sharing load among parallel AC generators/inverters proportional to their power ratings or operating costs. The technique relies on using small changes in voltage and frequency to dictate changes in real and reactive power levels. Optimal performance of droop control relative the Point of Common Coupling (PCC) is achieved when the relationship between voltage and frequency and real and reactive power is dictated by the following droop equations:
            f      -              f        0              =                            -                      k            p                          ⁢                  X          Z                ⁢                  (                      P            -                          P              0                                )                    +                        k          p                ⁢                  R          Z                ⁢                  (                      Q            -                          Q              0                                )                                U      -              U        0              =                            -                      k            q                          ⁢                  R          Z                ⁢                  (                      P            -                          P              0                                )                    +                        k          q                ⁢                  X          Z                ⁢                  (                      Q            -                          Q              0                                )                    
where f is frequency, U is fundamental voltage, kp and kq are the droop gains, P is real power, Q is reactance, X is the reactive impedance to the PCC, R is the real impedance to the PCC, and Z is the total impedance amplitude Z2=X22+R2. This requires knowledge of the X/R ratio of the impedance to the PCC.
The “virtual impedance droop control” technique is a time-domain implementation of droop control by which the converter is controlled to appear (from the perspective of the power grid) as a virtual AC voltage source in series with a virtual impedance having an inductive impedance (X) and a resistive impedance (R). This technique has several advantages including improved dynamic response and harmonic compensation. With this version of droop control, the above equations are satisfied when the X/R ratio of the virtual impedance is matched to the X/R ratio of the grid impedance. Existing techniques for implementing this matching involve injecting a current into the grid and analyzing the response. These techniques are computationally intensive, slow, and pose a risk causing instabilities in weak grids.
Therefore, there is a need in the art for an efficient technique for matching the ratio of the reactive impedance to the real impedance for a virtual impedance droop control technique.